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Bolometer-Based System Images Items at Terahertz Range

Photonics SpectraMay 2008
A technology developed to help ground-based telescopes discern tiny fluctuations in stellar radiation may also become useful for medical and security applications.

Scientists at the National Institute of Standards and Technology in Boulder, Colo., and at the University of Massachusetts Amherst have created a passive heterodyne imaging system that senses radiation in the 850-GHz range. The detector combines a hot electron bolometer with a monolithic microwave integrated-circuit low-noise intermediate-frequency amplifier, a harmonic multiplier local oscillator and off-axis parabolic mirrors.

Composed of a thin-film superconductor made of NbN sputtered onto a silicon substrate, the bolometer measures terahertz-scale radiation that is emitted by ordinary materials — including biological molecules and chemical agents — by mixing the radiation with a stable terahertz signal. The mirrors and a monolithic terahertz antenna couple the incoming local oscillator and terahertz signal into the bolometer. The researchers demonstrated that the imager has a thermal sensitivity of 0.4 K rms and a spatial resolution of ∼4 mm, which they expect to be improved substantially in the future.

The device presently acquires a 40 × 40-pixel image in about 20 min, but the investigators say that the acquisition time can be reduced to time spans suitable for medical imaging by integrating an array of the bolometers. For real-time imaging suitable for security-based scanning operations, an array of 100 bolometers should do, they believe. The researchers also say that the system can be adapted to concurrent spectroscopy functionality.

The researchers report their work in an upcoming IEEE Transactions on Microwave Theory and Techniques.